Vibration-sensitive measurement and test equipment require active or passive vibration isolation and damping. Thus, a device for vibration isolation must isolate shaking or vibrations of generally low amplitudes at an installation site from the measurement equipment. Also, such device is intended to dampen vibrations which are inherent to the equipment and which occur, for example, after a change in the load or the movement of mobile parts, and mostly have large amplitudes and low frequencies. In many applications, it is additionally necessary to compensate for load changes in the equipment by means of level control.
Commercially available pneumatic springs with level control are available for these requirements. These springs comprise a pneumatic spring for vibration isolation, a leveling screw for fixing the desired level of the object and a control loop with a pneumatic control valve for level control.
U.S. Pat. No. 4,057,212 describes, for example, such vibration isolation with level control using pneumatic spring elements and is incorporated by reference, herein. An adjustable, spring-preloaded leveling screw serves to set the level of the object. The leveling element is fastened, on the one hand, to the vibration-isolated object and, on the other hand, to the base. The conventional principle of vibration isolation is described in more detail below.
An object which is to be isolated from ground-borne vibrations and requires level control is usually fitted, for example, on a base plate, which rests on four pneumatic springs with level control. Since the position of the object is statically determined using three bearing points, it is possible for there to be only three control loops. For this reason, two pneumatic springs with level control are combined in terms of control equipment to form a single unit. By means of these three control loops, the position of the object, for example a measuring device or a microscope, is kept constant.
The use of pneumatic springs provides active, low-frequency vibration isolation in the event of ground-borne shaking. Good damping in the event of vibration inherent to the equipment is likewise achieved.
The pneumatic spring operates by using a volume of air as a compressible medium and a specially designated diaphragm to limit the volume of air. The air pressure required in the spring is dependent on the loading applied. For simple applications, the pneumatic springs are inflated by means of an air pump. In such systems, it is necessary to check the air pressure from time to time. For more complicated systems, the pneumatic springs are continuously connected to the compressed-air network. The desired air pressure is set at a fixed value via a restrictor valve using a manometer and an adjusting screw.
Schematically, the pneumatic spring comprises a lower part, an upper part, and a resilient air cushion situated between them. The lower part is positioned on a base, for example the floor of the building. The object which is to be isolated from the vibrations of the base is placed on the upper part.
In the event of displacements of a load on the object, the weight and pressure distribution change. Therefore the originally set level of the object changes as well. If this is undesirable, pure vibration isolation level control is required as an additional function. For this purpose, a level of the object above the base is determined. If the level at one of the measurement points changes owing to a displacement of a load on the object, this deviation is corrected by increasing or lowering the air pressure in the pneumatic spring assigned to the measurement point. The supply and removal of air are controlled by means of the level-control valve.
A valve lifter of a pneumatic level-control valve serves to determine the level between the object and the base. The pneumatic level-control valve is incorporated in the compressed-air feed line of the pneumatic spring and is mounted vertically from the outside at the base of the pneumatic spring. The valve lifter is arranged vertically and connected to the pneumatic level-control valve and is preloaded by a return spring. At the top, the return spring projects out of the pneumatic level-control valve. The position of the valve lifter controls the amount of compressed air fed to the pneumatic spring.
The desired level of the object mounted in an isolated manner can be set by means of an adjustable reference. To this end, a leveling screw is mounted vertically above the pneumatic level-control valve on the upper part of the pneumatic spring. After the pneumatic spring has been loaded by placing an object thereon, the leveling screw is rotated to set a desired level until its upper edge makes contact with the underside of the mounted object. The lower edge of the leveling screw is mechanically connected to the upper edge of the valve lifter arranged beneath it.
The level of the object is controlled by means of the pneumatic level control valve. To this end, the distance of the object from the base is determined by the valve lifter, which in this case operates as a mechanical gap sensor. Owing to the mechanical spring preloading, the valve lifter is in continuous flexible mechanical contact with the lower edge of the leveling screw arranged above it.
If the weight above an associated pneumatic spring rises due to a displacement of the load on the object, the valve lifter is forced downwards. As a result, a compressed-air feed line in the valve is opened, so that the pressure in the pneumatic spring rises and, as a result, the object is lifted. Under the influence of the return spring, the valve lifter follows until a desired level is reached and the compressed-air feed line is closed. If the weight above the pneumatic spring is reduced and the object is lifted, the valve lifter follows and thus opens an air discharge line, via which the air pressure in the pneumatic spring is reduced until the object has fallen back to the desired level and the valve lifter has closed the air discharge line. The pneumatic level-control valve thus balances the air pressure in the pneumatic spring as a function of the respective position of the valve lifter such that the level set by the leveling screw is restored.
The main drawback of the system lies in the use of a mechanical gap sensor in the form of the valve lifter. Due to the contact between valve lifter and leveling screw, the pneumatic level-control valve represents a mechanical coupling between the object mounted in a vibration-damped manner and the base of the pneumatic spring. Such coupling bridges the low-frequency pneumatic spring in the manner of a mechanical bypass.
Furthermore, the mass of the valve lifter and its return spring represent an oscillatory system which, in the event of external excitation (e.g., rapid rhythmic changes in load), can execute its own vibrations. This then leads to continual changes in height of the object applied.
Moreover, the control parameters of the level-control loop cannot be influenced by the user. In some cases, it is desirable to change the magnitude of the volumetric flow or response times, for example, if, owing to an unfavorable geometric arrangement of the bearing points, the independently operated control loops can affect one another and result in instability of the system, i.e., they can the build-up oscillation, or cause the vibration decoupling to become blocked.
The problems identified above are not intended to be exhaustive but rather are among many which tend to reduce the performance of the vibration isolation system. Other problems may also exist. However, those presented above should be sufficient to demonstrate that currently known solutions are amenable to worthwhile improvement.